This invention relates to the art of electric motors and more particularly to apparatus and method to be used in conjunction with a motor starting circuit for preventing start-up of a motor when its insulation resistance is of a value below a selected insulation resistance value below which the motor should no be energized.
It is known in the art that as a motor ages, the winding insulation resistance frequently decreases in value. For example, a new motor may have an insulation resistance on the order of 1,000 megohms. As the motor ages, the insulation deteriorates due to moisture, dirt and the like. In time, the insulation resistance may well decrease to a level below which the motor should not be energized. Thus, at some low level of insulation resistance, the application of a high voltage, such as 480 volts, may result in heating of the insulation causing vaporization of moisture resulting in the burning of the insulation. This may cause the resistance to further decrease which could well result in the current increasing to such a level that the metal employed in the windings may tend to melt, causing severe damage to the motor.
It is known in the art to periodically perform a test for measuring insulation resistance of a motor. The test is frequently referred to in the art as a megger test. In such a test, it is common to apply a fixed DC voltage, on the order of 500 volts, between the motor leads and ground. The insulation resistance is read directly from the meter in megohms. Such a megger test is typically performed by a technician employing a portable instrument to apply the test voltage and obtain a reading of the megohms while the motor is de-energized. Nothing is employed that will prevent an operator from starting such an electric motor that has recently been inspected and found to have a low megohm reading. It is desirable that instrumentation be provided as part of a motor starting circuit to prevent start-up of a motor whenever it is determined that the motor's insulation resistance is of a value below which the motor should not be energized.